Patent-family-89618818

A method of operating an obstacle warning system (4) for a wind turbine installation (2) comprises: operating a radar system (14) to obtain scan data for an area around the installation (2); analysing the scan data to identify a pair of signals comprising a first signal corresponding to a first location (34) and a second signal corresponding to a second location (32), the first location (34) being spaced from the second location (32); comparing the first signal and the second signal to determine a value indicative of the performance of the radar system (14) for an area including the second location (32); and activating the warning system (4) if the value is below a threshold.